Background: Paired box gene 2 (PAX2) encodes a transcription factor that plays an important role in embryogenesis with development of the mesonephric duct and urogenital tract. The utility of antibodies to PAX2 in the identification of tumors of renal origin especially at metastatic sites is well established. PAX2 is also utilized to confirm the diagnosis of nephrogenic adenoma in challenging cases. However, the expression of PAX2 in malignant germ cell tumors of the testes has not been well characterized.Design: A tissue microarray (TMA) was constructed from 56 malignant germ cell tumors. Ten cases composed of intratubular germ cell neoplasia (IGCN) and 7 cases composed of benign testicular tissue with unremarkable seminiferous tubules were also included in the TMA. Immunohistochemical stain for PAX2 was performed and analyzed. Only nuclear staining was interpreted as positive.Results: The various tumor types were stratified as follows; 29/56 (52%) cases were classic seminoma, 8/56 (14%) cases were yolk sac tumor, 8/56 (14%) cases were malignant teratoma, 8/56 (14%) cases were embryonal carcinoma, 2/56 (4%) cases were choriocarcinoma, and 1/56 (2%) cases was spermatocytic seminoma. Positive expression of PAX2 was identified in 9/29 (31%) cases of classic seminoma, 3/8 (38%) cases of yolk sac tumor, 2/8 (25%) cases of malignant teratoma, 2/8 (25%) cases of embryonal carcinoma and 1/2 (50%) cases of choriocarcinoma. PAX2 was negative in the spermatocytic seminoma case. PAX2 was also negative in all cases of IGCN and unremarkable seminiferous tubules.Conclusions: The expression of PAX2 in a number of malignant germ cell tumors of the testes may be due to the role of this transcription factor in the differentiation and development of the mesonephric duct and urogenital tract. Despite the obvious limitations of TMA based analyses of immunohistochemical stains, our study suggests that malignant germ cell tumors of the testes should also be included in the differential diagnosis of PAX2 positive metastatic tumors, especially in cases in which no renal mass is identified on imaging.Category: Genitourinary (including renal tumors)